Cathode-driven oscillator



CATHODE-DRIVEN OS C ILLATOR Y @RAY G. 02055)/ ATTORNEY.

Jan. 6, 1942.

M. G. CROSBY CATHODE-DRIVEN OS C ILLATOR Filed May 13, 1959 3Sheets-Sheet 2 A TTORNE Y.

Jan. 6, 1942.

M. G. CROSBY CATHODE-DRIVEN OS C ILLATOR Filed May 13, 1959 3Sheets-Sheet 3 INVENTOR. MURRAYG. CROSBY Patented Jan. 6, 1942CATHODE-DRIVEN OSCILLATOR Murray G. Crosby, Riverhead, N. Y., assignorto Radio Corporation of America, a corporation of Delaware ApplicationMay 13, 1939, Serial No. l273,460

(Cl. 25o-36) 12 Claims.

The present invention relates to oscillators and, more particularly, totwin type oscillators requiring only a two-terminal oscillator coil.

An object of the present invention is the generation of an alternatingcurrent wave of substantially perfect wave form and of substantiallyconstant amplitude over a wide band of frequencies.

Another object is the provision of a tunable oscillator giving an outputof substantially perfect wave form.

Still another object is the provision of a tunable oscillator requiringno change in regeneration with tuning.

A further object of the present invention is the provision of a simpletunable oscillator which does not require tapped or multiple coils forregeneration.

Another object is the provision oi an oscillator which places a reducedload on the tuned circuit.

My invention includes, among its features, a pair of tubes connected sothat the cathode circuit of one tube drives the cathode circuit of theother tube. A two-terminal tuned circuit is placed in the plate circuitof one tube and coupled to the grid circuit of the other tube. Theremaining plate and grid circuits may be directly connected to opposingsides of the power supply source.

The oscillator of the present invention has advantages over otheroscillators of the prior art, of which I am aware, in that only atwo-terminal tuned circuitl is required and that the grid coupled to thetuned circuit is a class A grid which does not damp the tuned circuit sothat stability and wave form are improved.

Further objects, features and advantages will appear from a reading ofthe following detailed description which is accompanied by drawings inwhich Figure 1 illustrates a circuit diagram of an embodiment of thepresent invention; Figure 2 illustrates a modification which is adaptedto produce square waves; Figure 3 illustrates an adaption of the presentinvention to an oscillator and a harmonic generator; Figure` 4illustrates a modification of Figure l utilizing linkcoupling circuits;Figure 5 illustrates a modification of Figure 3 utilizing crystalcontrol; Figure 6 illustrates a further modification of Figure l; Figure'l illustrates a modification of the present invention in which thecathode to ground capacity of the tubes is compensated; and Figure 8shows how a pair of oscillators according to Figure 1 may be combined sothat the wave 55 generated by one is modulated by the wave generated bythe other.

In Figure 1 of the drawings I have shown a pair of triode tubes. orelectron discharge structures T1 and T2 as being enclosed in the sameenvelope I. While I have shown the use of a twin triode it is to bedistinctly understood that my invention is not limited thereto but thata pair of entirely separate and distinct tubes may be used. It shouldfurther be understood that I intend by the use of the term tubes to meanstructures comprising a cathode electrode and an anode electrode withinan evacuated space, and means for causing, influencing or otherwisecontrolling an electron stream between said electrodes regardless ofwhether the evacuated space is common to more than one electrodeassembly or how it is maintained in an evacuated condition. Furthermore,the invention is not limited to the use of triode tubes since pentode orbeam tubes may be used.' Different types of tubes such as the triode anda beam tube may be combined. A common cathode resistance 3 is utilizedfor the cathodes of the two tubes T1 and T2 so that the voltage which isapplied to grid I2 of tube T1 is fed to the cathode circuit of tube T2.Utilizing the cathode circuit of tube T1 as an output circuit results inpercent inverse feedback and the voltage applied to the grid I2 of tubeT1 appears unamplifled on its cathode. Due to this high degree ofinverse feedback the grid I2 of tube T1 may be swung over a wide rangeof voltage without drawing grid current so that the impedance presentedto the tuned circuit, to be later described, by the grid is that of anegative biased or Class A grid which is very high. In order to producesustained oscillations the feedback voltage fed from the tuned circuit6, composed of inductance 'I and capacity 8, is fed through the grid I2of tube T1 through blocking condenser 4. This feedback voltage must befed back to the grid 22 of tube Tz with 180 degrees phase reversal inorder to produce oscillations. 'Ihis is readily accomplished due to thefact that the common cathode resistance 3 effects such reversal. Apositive voltage on the grid I2 of tube T1 causes more current to flowthrough the cathode resistor 3 and thereby makes both cathode II andcathode 2| more positive. The cathode 2I of tube T2 becoming morepositive is equivalent to the grid 22 becoming more negative. Thus, apositive voltage change on the grid I2 of tube T1 is equivalent to anegative voltage change on grid 22 of tube T2 and the required 180degree phase reversal is thus obtained. Tuned circuit 6 is tuned to therequired frequency of oscillation. The output signal is taken from themoving arm of potentiometer 2 to ground. The amplitude of the outputoscillations may be controlled by varying the position of the moving armof potentiometer 2 along the resistance. If desired, additionalamplitude control may be obtained when inserting a variable resistanceat the point marked X in Figure 1.

Plate I3 of tube T1 is directly connected to a source of plate potentialmarked +B. The grid 22 of tube T2 is directly grounded since the inputto this tube takes place through the cathode resistor 3. By-passcondenser is provided between the positive terminal of the source ofplate potential and ground.

In Figure 2 I have shown how the circuit of my invention may beconnected as a multi-vibrator to give a square wave form. This type ofmulti-vibrator has the advantage that its output wave form is squarewithout damped peaks on the tops of each half cycle as is characteristicof conventional multi-vibrators, of which I am aware. In this circuitresistances 26, 25 and condenser 24 control the frequency ofoscillation. but resistance 25 may be held at a fixed value so that thefrequency of oscillation is determined by the value of resistance26 andthe capacity of condenser 24. -The grid 22 of tube Ta is convenientlyavailable for the application of locking voltage by connecting themoving arm of potentiometer 28 to the grid 22. The desired lockingvoltage is applied tol terminals 21,`21 at the ends of the potentiometer24. The locking voltage may be a source of oscillations having afrequency which is a harmonic or a subharmonic of the desired frequencyof oscillations of the multi-vibrator. The square wave output from themulti-vibrator appears across resistance 29 and -is coupled to outputterminals 3|, 3| by means of condenser 30. The amplitude of the outputwave may be controlled by varying the position of the movable arm alongresistance 29.

In Figure 3 I have shown how the circuit according to my invention maybe adapted to a combined oscillator and harmonic generator. Tunedcircuit 6 oscillates at the fundamental frequency in the same way asdescribed with reference to Figure 1. The desired harmonie frequency istaken from tuned circuit 36, composed of inductance 31 and capacity 36.Tuned circuit 36 is tuned to the desired harmonic frequency. It shouldbe noted that the presence of this tuned circuit in the plate circuit oftube T1 does not eiect the operation at the fundamental frequencybecause of the fact that the impedance of 36 at a submultiple of itsfrequency will be very low. A high harmonic output is avallable fromtuned circuit 36 because the grid of tube Ti is driven by thefundamental oscillation circuit 6 over the full range of the fundamentaloscillations.

In Figure 4 I have shown a modification of the circuit shown in FigureV1 utilizing a linkcoupling circuit instead of a capacity couplingcircuit. Tuned circuit 46 has been substituted for the resistance 2 ofFigure 1. This tuned circuit comprises an inductance 41 and capacity 46,either or both of which may be variablevso that the circuit is tuned tothe same frequency as tuned circuit 6. In a variable inductive relationto inductance 1 and to inductance 41 are small coils 49, 49. The twocoils 49, 49, are connected together by leads 50. The circuit shown inFigure 4 has improved stability over the circui eviously shown due tothe greater selectivity obtainable by means of the coupled tunedcircuits.

A further modification of the form 0f the invention shown in Figure 3 isshown in Figure 5. The harmonic generator circuit of Figure 3 isarranged so that the fundamental circuit is crystal controlled.Piezo-crystal 68 takes the place of the fundamental tuned circuit 6. Aradio frequency choke B1 passes the necessary direct current to theplate of tube Tn. The harmonic output is taken from tuned circuit 36tuned to the desired harmonic in the same way as described withreference to Figure 3.

In Figure 6 I have shown an alternative method of coupling the cathodesof the two tubes I Ti and 'Iz together. In this figure the cathodes haveseparate resistors 63 and 64 coupled together by condenser 65. This formof construction enables individual adjustment of the cathode resistorsso that different biases may be placed in the two tubes. In some casescondenser 65 may be replaced by a resistor which serves the same purposeand provides an additional adjustmeni-l on the degree of feedback of theoscillator. By making 65 of a high reactance compared to the resistance64 a feedback is obtained which increases as the frequency is increased.With 65 replaced by an inductance the feedback increases as thefrequency is decreased. This helps to maintain a constant output as theoscillator is tuned over a wide range.

vAt Aultra-high frequencies the circuits, as previously described, maypresent the disadvantage that the cathode to ground capacity of the twocathodes presents a reactance which may be small compared to the commoncathode resistance at these higher frequencies. However, for normalvalues of cathode resistance the cathode reactance is not comparable tothe cathode resistance until a frequency of about 10 megacycles isreached. This disadvantage is not serious since the capacity may beneutralized by inserting, as shown in Figure 7, an inductance 66 inseries with a cathode resistor 3. By this means the capacity to groundof the cathodes may be tuned out in the same manner that televisionvideo amplifiers are equalized. Such equalization allows oscillation atthe higher frequencies and improves the stability by limiting the phaseshift due to capacity reactance across the cathode resistance. Thisphase shift otherwise causes the feedback to be different than exactlydegrees and would impair the stability for the higher frequencies. Withthe cathode to ground capacity of the tubes compensated for there is nodifficulty in making the oscillator, according to the present invention,oscillate at frequencies higher than 34 megacycles.

In Figure 8 I have shown a pair of oscillators as previously shown inFigure l and described with reference to that figure. However, the upperoscillator utilizing tuned circuit 6 is designed to oscillate at radiofrequencies and the lower oscillator having its output resistance 2connected in series with grid 22 of the upper oscillator is designed tooscillate at audio frequencies. This is accomplished by including intuned circuit 6' an inductance 1' of high reactance and/or a largecondenser 8'. The inductance 1 may conveniently be iron core choke suchas used for impedance coupling in an audio frequency amplifier. Thecircuit shown in Figure 8 is of particular advantage for testing radiosets and other service work since the radio frequency oscillator may betuned over a wide range of radio frequencies, depending on the constantsof tuned circuit 6 and the radio frequencies may be either modulated orunmodulated as desired. The percentage modulation may be varied within awide range by adjustment of potentiometer 2'. The output radiofrequencies, if modulated by the lower oscillator, may be modulated byany audio frequency within the audible range by a proper proportioningof the constants of tuned circuit 6'. Furthermore, any audio frequencywithin the audible range is also directly available at the terminals 9',l0'.

By using a circuit as shown in Figure 6 for radio service work, not onlymay the radio frequency circuits of a radio receiver be aligned andchecked but the audio response characteristics may be observed.

While I have shown and particularly described :several embodiments of myinvention, it is to be distinctly understood that my invention is notlimited thereto but that modifications within the scope of my inventionmay be made.

I claim:

l. An oscillator comprising a pair of electron discharge structures eachhaving an anode, a cathode and a grid, a frequency determining circuitcoupled to the anode of one of said structures and the grid of the otherof said structures, means for effectively maintaining the remaining onesof said anodes and grids at a fixed reference potential, and impedancemeans connected between said cathodes and a point of zero referencepotential for coupling said cathodes together.

2. An oscillator comprising a pair of thermionic discharge tubes eachhaving an anode, a cathode and a grid, a frequency determining circuitcoupled to the anode of one of said tubes and the grid of the other ofsaid tubes, means for coupling said cathodes together comprising acommon impedance connected from said cathodes to ground, an impedanceconnected from said separately mentioned grid to ground and an outputcircuit coupled to said last-mentioned impedance, said cathode couplingmeans serving to cause said cathodes to vary in potentials with respectto ground in like sense whereby said grids are effectively varied inpotential in phase opposition.

3. An oscillator comprising a pair of thermionic discharge tubes eachhaving an anode, a cathode and a grid, a resonant circuit coupled to theanode of one of said tubes, means for coupling the grid of the other ofsaid tubes to said resonant circuit, means for effectively maintainingthe remaining ones of said anodes and grids at a xed referencepotential, and means connected between said cathodes and a point of zeroreference potential for coupling said cathodes together.

4. An oscillator comprising a pair of thermionic discharge tubes eachhaving an anode, a cathode and a grid, a resonant circuit coupled to theanode of one of said tubes, means for coupling the grid of the other ofsaid tubes to said resonant circuit, means for effectively maintainingthe remaining ones of said anodes and grids at a xed referencepotential, and means for so coupling the cathodes of said tubes togetherthat the effective potential of both cathodes varies in a like sense,said last mentioned means comprising a common impedance connected fromsaid cathodes to saidl point of reference potential.

5. An oscillator comprising a. pair of thermionic discharge tubes eachhaving an anode, a cathode and a grid, a source of anode potential, aresonant circuit connected between said source and the anode of one ofsaid tubes. means for coupling the grid of the other of said tubes tosaid resonant circuit, a common impedance connecting the cathodes ofsaid tubes to ground, means for connecting the remaining grid to ground,means for connecting the remaining anode to said source and an outputcircuit connected from said rst mentioned grid to ground.

6. An oscillator and frequency multiplier comprising a pair ofthermionic discharge tubes each having an anode, a cathode and a grid, ameans resonant to a fundamental frequency connected to the anode of oneof said tubes, means for coupling the grid of the other of said tubes tosaid resonant means, impedance means connected between said 'cathodesand a point of zero reference potential for coupling the cathodes ofsaid tubes together, a circuit resonant to the desired harmonicconnected to the anode of the other of said tubes and an output circuitcoupled to said second resonant circuit.

7. An oscillator and frequency multiplier comprising a pair ofthermionic electron discharge structures each having an anode, a cathodeand a grid, a source of anode potential, means resonant to a fundamentalfrequency connected between the anode of one of said structures and saidsource, means for coupling the grid of the other of said structures tosaid resonant means, means for coupling the cathodes of said structurestogether, said means comprising a common impedance connected from saidcathodes to ground, a circuit resonant to the desired harmonic connectedbetween the anode of the other of said structures and said source and anoutput circuit coupled to said resonant circuit.

8. An oscillator and frequency multiplier comprising a pair ofthermionic discharge tubes each having an anode, a cathode and a grid, acrystal circuit resonant to a fundamental frequency coupled to the anodeof one of said tubes, means for coupling the grid of thepther of saidtubes to said crystal circuit, impedance means connected between saidcathodes and a point of zero reference potential for coupling thecathodes of said tubes together, a second circuit resonant to thedesired harmonic coupled to the anode of the other of said tubes and anoutput circuit coupled to said second resonant circuit.

9. An oscillator and frequency multiplier comprising a pair ofthermionic discharge tubes each having an anode, a cathode and a grid,a. source of anode potential, means for connecting said source to theanode of one of said tubes, a crystal circuit resonant to a fundamentalfrequency coupled to said anode, means for coupling the grid of theother of said tubes to said crystal circuit, means for coupling thecathodes of said tubes together comprising a common impedance connectedfrom said cathodes to ground, a second circuit resonant to the desiredharmonic connected between the anode of the other of said tubes and saidsource and an output circuit coupled to said second resonant circuit.

10. An ultra high frequency oscillator comprising a pair of electrondischarge structures each having an anode, a cathode and a grid, afrequency determining circuit coupled to the anode of one of saidstructures and the grid of the other of said structures, means foreffectively maintaining the remaining ones of said anodes and grids at afixed reference potential, means for so coupling said cathodes togetherthat the effective potential of both cathodes varies in a like sensecomprising a common impedance connected from said cathodes to said pointo! reference potential, an impedance connected from said separatelymentioned grid to xed reference potential and an output circuit coupledto said impedance, a portion of said common impedance being inductivewhereby the capacity of said structures to said point of referencepotential is neutralized.

ll. An oscillator comprising a pair of thermionic discharge tubes eachhaving an anode, a cathode and a grid, a resonant circuit coupled to theanode of one of said tubes, means for coupling the grid of the other ofsaid tubes to said resonant circuit, an impedance connected from each ofsaid cathodes to ground, and means for coupling the cathodes of saidtubes together, said last mentioned means comprising an impedanceconnected between said cathodes.

l2. An oscillator comprising a pair of electron discharge structureseach having an anode, a cathode and a. grid, a source of anodepotential, a resonant circuit connected between said source and theanode of one of said structures, an impedance connected from the grid ofthe other of said structures to ground, a condenser connecting said gridand said anode, a common impedance connecting the cathodes of saidstructures to ground, means for connecting the remaining grid to ground,means for connecting the remaining anode to said source and an outputcircuit coupled to said first mentioned impedance.

MURRAY G. CROSBY.

